Transferring video frames in telecommunication systems, such as video phones, video conference systems or Internet connections, is a demanding task due to the large amount of data needed to transfer a video frame, since the more bits are needed to transfer the data, the higher the data transmission rate must be. Various methods have been developed to solve this problem. To transfer a frame, the frame is usually divided into frame blocks whose size is selected to suit the system. Frame block information generally comprises information on the luminance, color and location of the frame block in the frame itself. The frame block data is compressed by each block using a desired coding method. The compression is based on removing the less significant data. Compression methods are divided primarily into three classes: spectral redundancy reduction, spatial redundancy reduction and temporal redundancy reduction. Various combinations of these methods are typically used in compression.
A YUV color model, for instance, is applied to spectral redundancy reduction. The YUV color model utilizes the fact that the human eye is more sensitive to changes in luminance than in chrominance, i.e. color. The YUV model has one luminance component (Y) and two chrominance components (U, V). The chrominance components are also sub-sampled. For instance, a luminance block according to the H.263 video coding standard is 16×16 pixels and both chrominance blocks, which cover the same area as the luminance block, are 8×8 pixels. The combination of one luminance block and two chrominance blocks is in this standard called a macro block.
To reduce spatial redundancy, a discrete cosine transformation (DCT), for instance, is used, in which the block to be compressed is preferably 8×8 pixels. In a DCT, the pixel presentation of a frame block is transformed into a space-frequency presentation. In addition, only the signal frequencies which exist in a frame block have high-amplitude coefficients, whereas the coefficients of the signals that do not exist in a frame block are close to zero. DCT is also a lossless transformation and interference is caused to the signal in quantizing only.
Temporal redundancy can be reduced by utilizing the fact that consecutive frames usually resemble each other, so instead of compressing each individual frame, motion data of the frame blocks is generated. The basic principle is as follows: an as good as possible reference block which has been coded earlier is searched for the frame block to be coded, the motion between the reference block and the frame block to be coded is modeled and the calculated motion vector coefficients are transmitted to the receiver. The difference between the block to be coded and the reference block is indicated as a prediction error component or frame. The problem is to find a reference block which produces as good a coding efficiency as possible (a sufficiently good picture quality with as small an amount of bits as possible) and a high calculation capacity, and thus also calculation time, required by the comparison.